A heat exchanger made of aluminum alloy (aluminum alloy heat exchanger) is mainly constituted of tubes, fins, and header pipes that are brazed to each other. Conventionally, extruded tubes having a surface thermally sprayed with Zn, fins composed of a (three layered) brazing sheet having clad layers of Al—Si alloy filler on both sides has been widely used in combination in the heat exchanger.
Recently, world-wide production of inexpensive, high-quality, and high-performance product has been realized utilizing a combination of a extruded tube with a surface coating for brazing composed of Si powder, Zn-containing flux, and binder, and fins made of a bare unclad sheet (single layer sheet that does not have a brazing filler).
In the heat-exchanger made of the latter combination, sacrificial anode fins containing Zn are used as the fins, and thereby suppressing occurrence and progress of corrosion of the tubes by sacrificial anode corrosion protection effect of the fins. In addition, Zn contained in the flux of the coating for brazing diffuses during brazing and forms a sacrificial anode layer on the surface of the tube. As a result, progress of corrosion generated on the tube is suppressed, and leakage of refrigerant due to corrosion of the tube is prevented.
In addition, during the brazing process, liquid brazing filler that is formed by reaction of the coating for brazing and the tube flows towards the joint of the fin and the tube and joins the fin and the tube by forming a fillet. Thus, high heat-exchanging performance can be obtained.
Based on the above-described background art, in Patent Reference 1, the inventors proposed a heat exchanger tube with a coating for brazing formed on the outer surface of the tube, where Si powder in an amount of 1 to 5 g/m2 and Zn-containing flux in an amount of 5 to 20 g/m2 were contained in the coating.
Since the Si powder and the Zn-containing flux are mixed in the coating of the proposed tube, Si powder fuses during brazing and forms a liquid brazing filler, and Zn in the flux diffuses in the liquid brazing filler and is spread uniformly on the surface of the tube. Since the diffusion rate of Zn in a liquid phase such as the liquid brazing filler is remarkably higher than the diffusion rate of Zn in solid phase, a substantially uniform Zn concentration is achieved on the surface of the tube. By this process, a uniform sacrificial anode layer is formed on the surface of the tube, thereby improving the corrosion resistance of the heat exchanger tube.